This invention relates to point of sale systems and more particularly to point of sale systems having an active and an inactive redundant system with read/write memories in both systems.
In some types of point of sale systems, for example the system described in U.S. Pat. No. 3,946,220, a plurality of key terminals are connected to a central processing unit which processes data received from the key terminals utilizing stored memory routines in a read only memory and dynamic data which is maintained in a read/write memory. The central processing unit computes totals and sums and other routines as called for at the key terminal stations and outputs data to those key terminal stations for direct reading by the operator. The central processing unit also prints out the totals at a central control module. Since the particular point of sale system to which this invetion is directed is utilized in the world's largest chain of fast food outlets, it is extremely important that a back-up system be provided in the event that the primary system fails during operation. In one such system, two identical systems are provided. One system is run in an active mode to service the key terminal. The other system is inactive and is simply maintained as a spare. In the event that the primary system fails, the spare system may be activated to take over the control and operation of the key terminal stations which are interconnected to both the active and inactive systems.
Although the read only memories of both systems contain the same data, the read/write memory of the active system, prior to shut-down, contained the most current data in relation to the key terminal stations. Such data would be data involved in actual transactions which were taking place at the time of the shut-down. Since it is desirable that the spare system, once it is activated, should take over the computations formerly handled by the other system, it is necessary for the spare system, when it becomes active, to operate from the most current and valid data which is available to it. Unfortunately, the spare system when it becomes active has no way of knowing whether the data in the read/write memory of the formerly active system is either valid or more current than the data in the read/write memory of the now active system. Thus, means must be provided for first of all determining which of the two read/write memories contains valid data, next which of the two read/write memories containing valid data also contains the most current data and then for transferring, if necessary, the most current valid data to the read/write memory which is to be used in the now active system.
Previous data processing systems of which the inventors are aware were not addressed to this problem. Typically, redundancy by way of memory is supplied simply by having both the active memory and the back-up memory operate in tandem. A disadvantage of this arrangement is that in the event of a failure in the active system, the memory in the back-up system will be simultaneously affected and possibly invalidated. This can result from a defective component in the active system just prior to shut-down, for example. Some other systems do not involve tandem memories, but these systems do not provide means for checking the back-up memory both as to the validity of its contents and as to the currency of the contents versus the currency of the contents of the memory which is no longer active.